Wide-band local networks which are being investigated at the present time, join into a single system all the communication services within a firm. Hence, they must allow not only conventional speech and data services, but also new services such as computer-aided design or manufacturing (CAD,CAM), video conferencing, remote control of production processes, etc.
While implementing such service integration, it is important to obtain high efficiency (in terms of optimal exploitation of the available band), high flexibility (i.e. easy adaptability to new service introduction and to changes in the characteristics of the various services and/or of individual subscriber requirements) and high performance (in terms of network capacity and service quality). Reliability and cost must also be taken into account.
The requirements connected with the integration are met if the information emitted by the various sources is organized into cyc ically repeating hybrid frames and if ordered access protocols are used, e.g. as described in commonly owned Italian Patent Application No. 67856-A/84, filed on Aug. 28, 1984 corresponding to U.S. application Ser. No. 768,861 filed Aug. 23, 1985, now U.S. Pat. No. 4,663,758 in the case of a folded unidirectional bus network.
Such an access protocol presents intrinsic reliability advantages, since a failure preventing a station from transmitting merely results in the nonuse of the access right and is interpreted as a renunciation of transmission, without affecting the access by the other stations.
As to reliability aspects which depend on the network topology, the ideal solution would be a reconfigurable network, i.e. a network that, in the presence of a failure on the line or in one or more stations, may assume a topology permitting the interconnection of all or most of the still operating stations, cutting off the faulty line trunk and the faulty stations; this, however, considerably increases the complexity of the access-handling devices and, in the case of a ring network, also requires duplication of the transmission subsystem. Generally speaking, the higher the reliability degree, the greater will be the network costs.
Assuming the use of an access and transmission scheme where hybrid frames are employed to meet service integration requirements, a reasonable compromise between network costs and reliability degree may be of interest for some applications.
An example of such compromise can be represented by the office automation, where temporary activity interruption might also be tolerable. In this case, non-duplicated structures (e.g. a unidirectional ring) may be used, offering maximum implementing simplicity, since supervision and sensing of the activity on the line are coincident with the reception. For safeguard against network failures, taking into account that in the indicated application, such failures chiefly interest terminals and their links to the main line (e.g. as a consequence of possible terminal displacements), the transmission line could be housed in a protected duct and bypasses could be provided to cut off branches involved in failures.
Usually ring networks are based on token-passing protocols and packet-switching information flows. However, performance is inadequate to handle communications with continuity characteristics, such as speech communications, in terms of both efficiency and variance of transmission delay. Thus access schemes have been proposed using cyclical hybrid frames even on a ring. In this case, the main problem is to avoid overlapping of the beginning of a frame with a residual of the preceding frame which is still to reach the destination. In fact, by contrast with the folded unidirectional bus, writing and reading channels in a ring coincide, and hence each frame queue and the beginning of a subsequent one might be present at the same time on the same line section.
An example of a solution to this problem is described by K. Hiyama, H. Narisawa e H. Satou in the paper "An Integrated Services Optical Fiber Local Area Network" .SIGMA. Newtork"", Hitachi Review, Vol. 32 (1983) No. 4.
The solution suggested here, provides a synchronous network in which the frames are of fixed length, the two frame regions are subdivided into intervals of equal and constant duration, and the overall signal-propagation delay in the network (delay comprising both propagation time along the fiber and the operating time periods in the individual nodes) is made equal to or an integral multiple of the frame duration. The individual channels are allotted to the communications in one-to-one correspondence, according to a TDMA technique.
In the example described, the frame has a duration of 125 s and comprises 400 channels, each conveying 10 bits, 8 information bits and 2 control bits. The transmission rate is 32 Mbit/s, so that each channel has a 64 kbit/s capacity (in terms of information bits).
A structure of this type is scarcely efficient and scarcely suited to handle communications with different band requirements.
In fact, owing to the short frame duration, frame and control signals form a considerable portion (about 20%) of the signals to be transmitted, thus impairing transmission capacity of the actual information signals. Besides, the packet switched communications, which generally use transmission lines with a band of few kbit/s, are allotted a 64 kbit/s channel, which then remains partly unexploited. Conversely, for wide-band, circuit-switched communications, multiple assignment algorithms are to be defined, and, as is known, the higher the performance required, the greater is the complexity of such algorithms.
Finally, the network implementation is complicated by the strict relations between the frame length and the propagation delay. As a result, such a network is scarcely flexible, because a complete reorganization of frames and channels is required in case of architectural modifications.